1. Field of the Invention
The present invention relates to a process for producing a fluorine-containing graft copolymer, and more particularly to a process for producing a thermoplastic fluorine-containing graft copolymer.
2. Description of Related Art
Fluorine-containing copolymers are polymeric materials having a distinguished resistances to heat and chemicals. Above all, fluorine-containing copolymer elastomers have been widely used as sealing materials for O-rings, packings, oil seals, etc. or as molding materials for hoses, etc., calling for a heat resistance and an oil resistance. However, vulcanization products of the fluorine-containing copolymer elastomers have a difficulty in recycling of burrs and scraps due to their insolubility and non-meltability. Furthermore, the vulcanization process itself requires an enormous amount of energy and a precise process control.
Thermoplastic elastomers, on the other hand, are easy about recycle of burrs and scraps because of possible omission of vulcanization step itself and thus can be said as materials suitable for nowadays social demands such as resource saving and low waste discharge. Furthermore, the thermoplastic elastomers have a self-reinforcability and thus basically do not require any addition of a reinforcing filler. That is, the thermoplastic elastomers can be appropriately used in applications, where the filler pollution is a problem, such as fields of medicine, food, semiconductor device production, etc.
So far known thermoplastic fluorine-containing copolymer elastomers are disclosed typically in the following documents:
JP-A-53-3495 discloses production of a multi-segmented polymer, at least one of whose polymer chain segments is a fluorine-containing polymer chain segment, as shown by the following formula: ##STR1## where X: residue freed from iodine of an iodide compound, and
A, B, . . . :polymer chain segments, by successively polymerizing monomers, which are to constitute respective polymer chain segments so as to form at least two kinds of polymer chain segments between the carbon-iodine bond of the iodide compound when at least two kinds of radical-polymerizable monomers are polymerized in the presence of the iodide compound having a carbon-bonded iodine. The resulting multi-segmented polymer has a good rubber resiliency, but does not always satisfy the compression set characteristics, which is a practically important characteristics of sealing materials.
JP-A-61-49327 discloses a fluorine-containing segmented polymer, which comprises a chain of 2 or 3 polymer chain segments and, an iodine atom at one end of the chain and a residue freed from at least one iodine atom of an iodide compound at the other end of the chain, where one kind of polymer chain segment (in case of the chain consisting of two polymer chain segments) or one or two kinds of polymer chain segments (in case of the chain consisting of three polymer chain segments) is an elastomeric polymer chain segment having a molecular weight of 30,000 to 1,200,000, selected from (1) VdF/HFP/TFE (molar ratio=45-90:5-50:0-35) polymer and (2) FAVE/TFE/VdF (molar ratio:15-75: not more than 85:0-85) polymer, and the remaining polymer chain segment or segments are non-elastomeric polymer chain segments having a molecular weight of 3,000 to 400,000, selected from (3) VdF/TFE (molar ration=0-100:100:0) polymer and (4) ethylene/TFE (molar ratio=40-60:60-40) polymer, a ratio by weight of the elastomeric polymer chain segment to the non-elastomeric polymer chain segment being 40-95:60-5. The fluorine-containing segmented polymer also has a distinguished rubber resiliency, but does not always satisfy the compression set characteristic, which is a practically important characteristic of sealing materials. The foregoing abbreviations correspond to the following compounds:
VdF: vinylidene fluoride PA1 HFP: hexafluoropropene PA1 TFE: tetrafluoroethylene PA1 FAVE : perfluoro(alkyl vinyl ether) having a C.sub.1 -C.sub.3 alkyl group
JP-A-58-206615 discloses production of flexible fluoro resins by preparing a fluorine-containing elastic copolymer having a glass transition temperature equal to room temperature or less at the first stage by copolymerizing at least one kind of monomers including at least one kind of fluorine-containing monomers with a monomer having a double bond and a peroxy bond simultaneously in the molecule, thereby introducing the peroxy group into the copolymer molecule, and graft copolymerizing the copolymer obtained at the first stage in an aqueous emulsion or a dispersing solvent with at least one kind of monomers including at least one kind of fluorine-containing monomers, capable of giving a crystalline polymer having a melting point of 130.degree. C. or higher at the second stage.
The fluoro resins obtained by the process require no crosslinking treatment and can be easily molding-processed to give molding products having good mechanical characteristics and proper applicability as sealing materials, but their specific embodiments typically as given in Example 1 show the necessity for a series of complicated steps of emulsion polymerization--salting out water washing-solvent washing-drying-graft polymerization (solution polymerization--solvent separation drying. Thus, the process is not preferable from the viewpoint of production cost.
The present inventors, previously proposed a fluorine-containing graft copolymer, which was a thermoplastic fluorine-containing copolymer elastomer having an improved compression set characteristic and properly applicable as molding materials for sealing materials or the like, prepared by copolymerizing at least two kinds of monomers selected from the group consisting of fluorinated monomers and propylene in the presence of (A) a saturated, iodine-containing compound represented by the following general formula: EQU RIn
where R is a fluorohydrocarbon group, a chlorofluorohydrocarbon group, a chlorohydrocarbon group or a hydrocarbon group and n is 1 or 2, or an iodine and bromine-containing compound represented by the following general formula: EQU InBrmR
where R is a fluorohydrocarbon group, a chlorofluorohydrocarbon group, a chlorohydrocarbon group or a hydrocarbon group and n and m are each 1 or 2, and (B) an unsaturated fluorohydrocarbon or an unsaturated hydrocarbon, each of which contains an iodine atom or a bromine atom, and then graft polymerizing the resulting copolymer with at least one kind of monomer selected from the group consisting of fluorinated monomers and ethylene (JP-A-10-130341).
However, the thus obtained fluorine-containing graft copolymer has a melting point Tm of about 160.degree. to about 280.degree. C. and is not satisfactory for application to molding materials for sealing materials requiring a higher heat resistance. In other words, among the resistance to chemicals and the heat resistance (oxidation resistance) due to the C--F bonds of fluorine-containing copolymer the heat resistance characteristics has not been fully satisfied yet.
On the other hand, JP-A-7-316246 discloses thermoplastic elastomers having a distinguished compression set characteristic. The thermoplastic elastomers have a block structure composed of at least one of elastic type A fluorinated polymer segments and at least one of plastomeric type B fluorinated polymer segments, at least one of type A or type B segments being composed of a unit derived from an olefin iodide represented by the following general formula: EQU CH.sub.2 .dbd.CXRfCHRI
where X: H, F or CH.sub.3 and Rf: (per)fluoroalkylene group or (per)fluoropolyoxyalkylene group.
Even such thermoplastic elastomers have a melting point Tm as low as about 165.degree. C., and furthermore the olefin iodide represented by the forgoing general formula has a high production cost and their compression set (120.degree. C., 24 hr.) is about 53%. Thus, the thermoplastic elastomers can be said to have no such properties as to meet the expended costs.